This invention relates generally to pyrotechnic squibs and, particularly, to a pyrotechnic squib having an improved bridgewire circuit that addresses both all-fire and no-fire requirements.
In an electrically controlled explosive system, including a pyrotechnic system, an electro-explosive device translates an electrical signal into a pyrotechnic signal for selectively beginning the detonation of an explosive material. Depending on the particular industry in which they are used, such devices are referred to by various names including squibs, initiators, ignitors and electric matches. Moreover, the pyrotechnic signals provided by the devices may take several forms (e.g., gas pressure, a flame front or a shock wave) depending on the particular use. As used herein, the term "squibs" refers to electro-explosive devices collectively.
A conventional squib includes a bridgewire that heats up in response to an electrical current. In turn, the heat generated by the bridgewire initiates detonation. The bridgewire is a resistive component, such as a wire or filament, coated or otherwise in contact with a flammable or explosive composition. This pyrotechnic composition is typically the first in a sequence of compositions of decreasing sensitivity, increasing mass and increasing energy (i.e., a pyrotechnic or explosive train). Typically, the bridgewire components are enclosed in a metal, plastic, or paper housing for maintaining the proper juxtaposition of the components. The housing also protects the pyrotechnic composition from humidity and other environmental effects. A squib operates when the current heats the bridgewire until it reaches a temperature high enough to start a chemical reaction (e.g., burning or exploding) in the first composition of the pyrotechnic train. It is to be understood that the bridgewire need not be in the actual form of a wire and may be a metal, an alloy (e.g., nichrome or tungsten) or another conducting material (e.g., semiconductor).
One shortcoming of presently available squibs is their inability to simultaneously meet fairly precise all-fire and no-fire specifications. All-fire requirements specify a minimum current and duration at which all squibs of a particular design are expected to fire (i.e., ignite to begin detonation). On the other hand, no-fire requirements specify a maximum current and duration that can be applied to the particular squibs without causing them to activate. A squib is needed that is sensitive enough to meet the former all-fire requirements but insensitive enough to meet the latter no-fire requirements. The problem is complicated by the fact that the response of a conventional squib varies with temperature, pressure, acceleration and other environmental factors. Presently available squibs can be, for example, sufficiently sensitive to meet all-fire specifications at a minimum required operating temperature but too sensitive to meet no-fire specifications at a maximum operating temperature.
Squibs are useful in ignitors, pin pullers, pin pushers, wire cutters, exploding nuts, explosive bolts, detonators, explosive bolts, rocket motors, gas generators, thermal batteries, signal flares, safe-and-arm apparatus, pressure cartridges, pyro switches, pyro valves, bellows actuators, piston actuators, perforators, air bag inflators, seat belt tensioners, and the like.